Solarprod

Shortfall (off-grid)

The kWh of load the system couldn’t cover from solar plus battery without dropping below the battery’s minimum reserve. Real-world consequence: the lights dim, the fridge cycles off, or the inverter trips because the battery hit its low-voltage cut-off. Each shortfall row in the Action plan is a day you’ll need a fallback plan for.

Where the number comes from

The simulator chains battery state from one day to the next: the charge at the end of one day is the charge at the start of the next morning. Each step it tries to serve load from (in priority order) live solar, then battery discharge. If neither can cover load — and battery has already dropped to its configured minimum reserve — the unmet portion is recorded as shortfall and carried into the row total.

The minimum reserve is critical here: it’s the charge below which you don’t want to discharge, either to protect the battery chemistry (lithium) or because deeper cycling shortens lifespan dramatically (lead-acid). The simulator respects your configured reserve strictly — once the battery reaches it, the battery stops supplying load even if the rig is calling for power.

What to do on a shortfall day

For caravan / rv setups

  • Move to a powered site. A caravan park stay for one night plugs you into 230 V / 120 V shore power, lets the battery refill via the on-board charger, and powers everything from the grid for the day. Cheapest fix when you have one nearby.
  • Run a generator. Petrol or diesel inverter generator (Honda EU22i, Yamaha EF2200iS, Champion 2000W class) makes about 1.5–2 kWh per litre of fuel. A 2–3 hour run while parked usually closes the gap on a single shortfall day. Run during the day while you’re out walking so the noise isn’t on top of you.
  • Reduce non-essential loads. Turn off the inverter when you’re not using it, skip the induction cooktop and use gas, defer laundry / charging until a headroom day, run the air-con in fan-only mode.
  • Move your itinerary. If the shortfall day is partway through a stay at a remote site, consider leaving a day early and putting the stay night somewhere with shore power instead.

For off-grid home setups

  • Fire the backup generator. Long enough to top the battery up to a comfortable level and cover the day’s evening loads. Bigger generators (5–10 kW) make this a 1–2 hour exercise; smaller generators (2–3 kW) need to run longer.
  • Shift heavy appliances — washing machine, dishwasher, oven, water-heater top-up — to a known-headroom day later in the forecast.
  • Skip the optional loads — workshop tools, pumping the dam, EV charging — until solar recovers.
  • Wood / propane substitution. Heat with wood, cook on propane, run hot water from a gas instant or wetback for the day.

When shortfall is happening too often

A row of shortfall rows in your forecast — three or four out of seven days — means the system is undersized for the load you’ve told it about. Three levers:

  • More panels. Cheapest per kWh of extra capacity. Useful especially in winter latitudes where short days are the binding constraint.
  • Bigger battery. Lets you carry surplus from a clear day through to the next cloudy one. Most useful in temperate climates with mixed cloud cover.
  • Less load. Insulation, gas / wood substitution, removing inverter idle, more efficient fridge. Cheapest if your current load has obvious slack.

Run the simulator with each scenario to see which combination drops the shortfall column to all dashes.

Closes this tab and returns to the calculator.